1. Field of the Invention
The invention relates to aqueous solutions comprising high concentrations of baclofen in the range of greater than 2.0 mg/mL up to about 10 mg/mL. These solutions can be used as pharmaceuticals. The invention also relates to various methods of preparing stable solutions in this concentration range.
2. Background
Baclofen is a skeletal muscle relaxant and antispastic agent. Baclofen is a structural analog of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), and may exert its effects by stimulation of the GABAB receptor subtype. Baclofen is the generic (USAN) name (USP Dictionary of USAN and International Drug Names 2003) for 4-amino-3-(p-chlorophenyl) butyric acid, a derivative of γ-aminobutyric acid. Its structural formula is:

Baclofen is a white to off-white, odorless or practically odorless crystalline powder, with a molecular weight of 213.66. It is slightly soluble in water, very slightly soluble in methanol, and insoluble in chloroform. Its slight solubility in water makes it difficult to obtain stable aqueous solutions of baclofen that have concentrations greater than approximately 2 mg/mL.
Baclofen can be administered orally, but when injected directly into the intrathecal space of a patient effective CSF concentrations are achieved with resultant plasma concentrations 100 times less than those occurring with oral administration. Baclofen injections (Lioresal Intrathecal, Medtronic) are therefore commonly administered intathecally via an implanted pump to manage severe spasticity of spinal cord origin. (McEvoy, 2003) Presently, baclofen is commercially available for injection as a 2 mg/mL solution having a pH of 5 to 7 and the following simple preservative-free formula (Lioresal Intrathecal package insert):
Baclofen2 mgSodium chloride9mgWater for injectionqs 1mL
Unfortunately, the 2 mWmL concentration has been inadequate to control the pain and symptoms of some patients. An additional difficulty is that mixing a 2 mg/mL baclofen injection with other drugs such as morphine or hydromorphone in “cocktails” to aid in control of pain can dilute the baclofen content to unacceptably low levels. In these and other applications, it can be desirable to administer a more concentrated solution of baclofen in order to reduce the volume of baclofen solution to be administered. But due to baclofen's slight solubility in water, aqueous solutions having a higher concentration of baclofen have not been shown to be commercially viable products. At higher concentrations baclofen may not entirely dissolve in aqueous solution, or it may have an unacceptable tendency to precipitate out of solution during storage.
An upper limit on room temperature aqueous solubility of baclofen has been reported by some sources as 4.3 mg/mL (Abuja, 1985), however that concentration was achieved by allowing the baclofen to dissolve over a period of weeks or months until it reached an equilibrium state. In an equilibrium solution there is always particulate baclofen present. The solution is at equilibrium because the rate at which the baclofen particulates dissolve is equal to the rate at which the dissolved baclofen precipitates out of solution.
Baclofen has not been shown to be nearly as soluble when less time-consuming methods are used to dissolve it, nor has it been reported that concentrations equaling or even approaching the 4.3 mg/mL concentration can be achieved in solutions that are not at equilibrium, i.e. where no particulate baclofen is present. Commercially and pharmaceutically acceptable baclofen solutions must not contain any significant amount of particulates, and the baclofen must stay in solution without precipitating prior to and during administration to a patient. Only solutions of baclofen having a concentration of 2 mg/mL have previously been demonstrated to have the properties that make them an acceptable commercial pharmaceutical product by remaining stable for extended periods of time without precipitation of significant amounts of baclofen particulates.
Various sources have reported stable suspensions or syrups of baclofen for oral administration that had concentrations even higher than equilibrium concentration of 4.3 mg/mL, (Allen et al., 1996; Johnson and Hart, 1993) but none of these preparations are acceptable for pharmaceutical injection uses. Other sources have reported the production of microspheres containing 12 to 50% baclofen (Cruaud et al., 1999, but such microspheres are not compatible with delivery systems requiring an aqueous solution of baclofen.
It was also known that baclofen can be readily dissolved in very high and very low pH solutions. For example, Ahuja (1985) reported that concentrations of baclofen greater than 20 mg/ml could be obtained by dissolving baclofen in aqueous solutions of 0.1N HCl or aqueous solutions of 0.1N NaOH. Significantly, the pKa1 for baclofen at 20° C. is reported by Ahuja to be 3.87±0.1 and the pKa2 at 20° C. 9.62±0.1. The pH of a 0.1N HCl solution is well below the pKa1 value for baclofen and the pH of a 0.1N NaOH solution is well above the pKa2 value for baclofen. Thus baclofen at these pHs would be expected to have different solubility properties than baclofen dissolved in more neutral pH solutions. Moreover, it would be expected that high concentration solutions of baclofen that were prepared at pHs below pKa1 or above the pKa2 value of baclofen would tend to fall out of solution when the pH of the solution was adjusted to a value between the pKa1 and pKa2 values. Because solutions having a pH of less than 4 or greater than 8.5 would not expected to be acceptable pharmaceutically, the use of very high or very low pH solutions of baclofen does not resolve the need for higher concentration aqueous solutions of baclofen.
Therefore, there appears to be a clinical need for more concentrated aqueous solutions of baclofen having acceptable pharmaceutical properties, and most preferably for concentrated solutions that are also stable in a variety of storage conditions and for extended periods of time.